Answer this question truthfully
### Question: What causes Hairy cell leukemia?
### Response: HCL is caused by the abnormal growth of B cells. The cells look "hairy" under the microscope because they have fine projections coming from their surface.
HCL can lead to low numbers of normal blood cells.
The cause of this disease is unknown. It affects men more often than women. The average age of diagnosis is 55.